1. Field of the Invention
The present invention relates to methods for producing a copper foil, and more particularly, to a method for producing a porous copper foil.
2. Description of Related Art
Recently, notebook computers, mobile phones, PDA are developed to be minimized and lightened, and batteries thereof thus need to be minimized Nonaqueous electrolyte secondary batteries, such as lithium ion secondary batteries, have high energy density and high capacity, and thus are widely used in the above mentioned electronic devices.
In lithium ion secondary batteries, lithium transition-metal composite oxide used as material of a positive electrode (cathode) and carbon material such as graphite used as a negative electrode (Anode) are respectively mixed with a conductive agent and a binder to form a paste for coating respective collectors such as an aluminum foil and a copper foil, so as to form a positive electrode and a negative electrode. A separator is disposed between the two electrodes to form a battery set, and electrolyte solution is injected into the battery set to form a lithium ion secondary battery. While recharging the battery, the lithium ions in the material of the positive electrode move to the material of the negative electrode. However, while discharging the battery, the lithium ions in the material of the negative electrode move to the crystal structure of the positive electrode. When the cycle of charge/discharge repeats, the lithium ions exchange between the positive electrode and the negative electrode. Therefore, it is critical to efficiently use lithium ions in the battery for high performance lithium ion secondary batteries.
Generally, an electrolytic copper foil or a rolled copper foil is used as a collector of a negative electrode in a lithium ion secondary battery. The electrolytic copper foil and the rolled copper foil both have two flat surfaces for carbon material to be evenly coated thereon. When the thickness of the coated carbon material on the copper foil is not even, the usage efficiency of the active substance (i.e. lithium ions) is poor, and thus the capacity of the battery is reduced. To address this issue, a porous copper foil is used in the prior art, in which the copper foil having through-pores made in a direction of thickness of the foil, such that lithium ions freely move through both sides of the copper foil, and thus lithium ions are efficiently used and capacity of the battery is increased. In addition, for the same battery volume, the porous copper foil has increased surface area for accommodating more carbon material, such that the battery has more capacity. The adhesiveness between the porous copper foil and the carbon material is increased due to the increased contact area between the porous copper foil and the carbon material, such that after charge/discharge of the battery, the carbon material is not easily peeled, and thus the battery maintains high capacity and has extended life. Hence, to efficiently use lithium ions in lithium ion secondary batteries, a porous copper foil with through-pores is used as a collector of a negative electrode.
Japanese Patent No. 10-112326 discloses a method for forming a copper foil having a porous structure, in which a stainless tube coated with a polyester felt is disposed on a titanium drum; the polyester felt contacts the surface of the drum; a PVC tube is disposed on the stainless tube; sulfuric acid solution is directed to the polyester felt through the opening of the PVC tube; voltage is applied between the stainless tube and the drum to electrolyze the sulfuric acid solution for oxidizing titanium on the surface of the drum to form a titanium oxide film; electroplating is performed on the drum to form a copper foil; and the copper foil is continuously stripped so as to form an electrolytic copper foil with a porous structure.
Japanese Patent No. 10-112326 also discloses various cathode oxidation devices such as the polyester felt for accommodating sulfuric acid to make the sulfuric acid contact the drum, a tank for accommodating an acidic solution, a bottle for accommodating an acidic acid, and a container having a nozzle for spraying an acidic acid. Further, voltage is applied between each of the above devices and the drum for oxidizing the drum to form an oxide film with a thickness of at least 14 nm, and then electroplating is performed to form a porous copper foil.
Taiwanese Patent No. 428049 discloses a method for forming a copper foil having through-pores, wherein a surface of a cathode is treated by coating with lipid, electroplating with copper pyrophosphate, or applying voltage with an acidic solution to form an oxide film; electrolysis of copper is formed on the treated surface of the cathode; and the cathode is stripped to obtain a copper foil. Further, Taiwanese Patent No. 428049 discloses that the difference in roughness (Rz) of both sides of the copper foil is 5-20 μm, and the copper foil thus has good adhesiveness with active substance while being used as a collector in a secondary battery.
While forming a lithium ion secondary battery, active materials of a positive and a negative electrodes are respectively mixed with a conductive agent and a binder to form a past material for being coated on corresponding collectors (such as an aluminum foil and a copper foil). It is known in the art that when both sides of the copper foil have similar roughness and uniformity, the resulting lithium ion secondary battery has better performance. On the contrary, when both sides of the copper foil have significantly different surface profiles, the both sides of the copper foil have different roughness and uniformity, and the resulting lithium ion secondary battery has reduced performance.
It is clear that the usage of porous copper foil overcomes the reduced performance resulting from the uneven coating of carbon material. However, the formation of a porous copper foil usually results in the increased roughness Rz of Matte side (surface of the copper foil without contacting the drum cathode); and when there is a severe difference in the roughness Rz between the Matte side and Shiny side (surface of the copper foil contacting the drum cathode), uneven coating of carbon material occurs and thus performance of the lithium ion secondary battery increased by the porous copper foil is offset. Therefore, there is a need to develop a method for forming a porous copper foil having great flatness and similar roughness of both sides so as to overcome uneven coating of carbon material and to achieve efficient usage of lithium ions.
In addition, the conventional method for forming a porous copper foil is complicated, wherein a thin copper layer is formed on the surface of a cathode by electrolysis, and the surface of the cathode is treated by coating with lipid or applying voltage with an acidic solution to form an oxide film on the surface of the cathode. Moreover, the residual lipid or residual oxide film may influence the surface quality of the copper foil, and further affect efficacy of the secondary battery. Hence, there is a need to develop a method for forming an improved porous copper foil.